FIG. 1 is a block diagram illustrating an example of a conventional ring-type broadband communications system, such as a two-way hybrid/fiber coaxial (HFC) network. It will be appreciated that other networks exist, such as a star-type network. These networks may be used in a variety of systems, including, for example, cable television networks, voice delivery networks, and data delivery networks to name but a few. The broadband signals transmitted over the networks include multiple information signals, such as video, voice, audio, and data, each having different frequencies. Headend equipment included in a signal source, or a headend facility 105, receives incoming information signals from a variety of sources, such as off-air signal source, a microwave signal source, a local origination source, and a satellite signal source and/or produces original information signals at the facility 105. The headend 105 processes these signals from the sources and generates forward, or downstream, broadcast signals that are delivered to a plurality of subscriber equipment 110. The broadcast signals can be digital or analog signals and are initially transported via optical fiber 115 using any chosen transport method, such as SONET, gigabit (G) Ethernet, 10 G Ethernet, or other proprietary digital transport methods. The broadcast signals are typically provided in a forward bandwidth, which may range, for example, from 45 MHz to 870 MHz. The information signals may be divided into channels of a specified bandwidth, e.g., 6 MHz, that conveys the information. The information is in the form of carrier signals that transmit the conventional television signals including video, color, and audio components of the channel. Also transmitted in the forward bandwidth may be telephony, or voice, signals and data signals.
Optical transmitters (not shown), which are generally located in the headend facility 105, convert the electrical broadcast signals into optical broadcast signals. In most networks, the first communication medium 115 is a long haul segment that transports the signals typically having a wavelength in the 1550 nanometer (nm) range. The first communication medium 115 carries the broadcast optical signal to hubs 120. The hubs 120 may include routers or switches to facilitate routing the information signals to the correct destination location (e.g., subscriber locations or network paths) using associated header information. The optical signals are subsequently transmitted over a second communication medium 125. In most networks, the second communication medium 125 is an optical fiber that is typically designed for shorter distances, and which transports the optical signals over a second optical wavelength, for example, in the 1310 nm range.
From the hub 120, the signals are transmitted to an optical node 130 including an optical receiver and a reverse optical transmitter (not shown). The optical receiver converts the optical signals to electrical, or radio frequency (RF), signals for transmission through a distribution network. The RF signals are then transmitted along a third communication medium 135, such as coaxial cable, and are amplified and split, as necessary, by one or more distribution amplifiers 140 positioned along the communication medium 135. Taps (not shown) further split the forward RF signals in order to provide the broadcast RF signals to subscriber equipment 110, such as set-top terminals, computers, telephone handsets, modems, televisions, etc. It will be appreciated that only one subscriber location 110 is shown for simplicity, however, each distribution branch may have as few as 500 or as many as 1000 subscriber locations. Additionally, those skilled in the art will appreciate that most networks include several different branches connecting the headend facility 105 with several additional hubs, optical nodes, amplifiers, and subscriber equipment. Moreover, a fiber-to-the-home (FTTH) network 145 may be included in the system. In this case, optical fiber is pulled to the curb or directly to the subscriber location and the optical signals are not transmitted through a conventional RF distribution network.
In a two-way network, the subscriber equipment 110 generates reverse RF signals, which may be generated for a variety of purposes, including video signals, e-mail, web surfing, pay-per-view, video-on-demand, telephony, and administrative signals. These reverse RF signals are typically in the form of modulated RF carriers that are transmitted upstream in a typical United States range from 5 MHz to 40 MHz through the reverse path to the headend facility 105. The reverse RF signals from various subscriber locations are combined via the taps and passive electrical combiners (not shown) with other reverse signals from other subscriber equipment 110. The combined reverse electrical signals are amplified by one or more of the distribution amplifiers 140 and generally converted to optical signals by the reverse optical transmitter included in the optical node 130 before being transported through the hub ring and provided to the headend facility 105.
FIG. 2 is a block diagram of one branch in a conventional communications system 200. In the conventional network 200, the signals may be transmitted in a Moving Pictures Experts Group (MPEG) transport stream format. The signals are modulated with a bandpass modulator 210. The bandpass modulator 210 frames, encodes, and modulates the MPEG signals in a known manner. The modulation scheme may be quadrature amplitude modulation (QAM) with a 64-QAM or 256-QAM transport stream format. Subsequently, the modulated signals are transmitted over a transmission segment 215, which can be optical fiber, waveguides, coaxial cable, or free space. A plurality of receivers 220 subsequently demodulate the modulated signals in a known manner to recover the originally transmitted signals, where the demodulator typically includes a QAM demodulator, a decoder, and MPEG framing equipment. Only one transmission segment 215 coupled to the plurality of receivers 220 is shown, however, it will be appreciated that there are typically several distinct transmission segments each coupled to a bandpass modulator 210.
FIG. 3 illustrates a communications system 300 that includes the conventional communications system 200 and a baseband transport segment 305. The communications system 300 is a first improved system of the conventional communications system 200. The signal source 205 preferably provides MPEG transport stream signals and, prior to bandpass modulating, transports the signals via the baseband communications transport segment 305, which in this improved system can be, for example, SONET, RPR IEEE 802.17, or Ethernet 10/100/T segments. Subsequently, the bandpass modulator 210, which is typically enclosed within a hub or node located at the remote end of the baseband transport segment 305, receives the baseband signals and creates bandpass signals that are then transmitted via the transmission segment 215 to the plurality of receivers 220.
Advantageously, the communications system 300, as illustrated, allows for greater distances between the signal source 205 and the plurality of receivers 220 than the communications system of FIG. 2. Furthermore, the baseband transport segment 305 does not introduce any degradation to the source signal, and may be regeneratively repeated to produce a cascade of segments, thereby further extending the distance between the signal source 205 and receivers 220. Additionally, the baseband transport segment bandwidth is essentially the same as the bandwidth of the source signal. It will be appreciated that compression techniques may be employed to reduce the required bandwidth. Disadvantageously, however, the communications system 300 requires a network topology that may be inappropriate for certain applications. For example, broadcast or multicast applications that require the source signal to be transmitted to multiple geographically distinct receivers through multiple baseband transport segments require additional bandpass modulators at the remote end of every baseband transport segment, thereby increasing the system management and expense.
FIG. 4 illustrates a communications system 400 that is a further improved transport system. The communications system 400 utilizes a digital bandpass transport system 402 in the middle of the conventional communications system 200. The communications system 400 includes the signal source 205 where the signals are subsequently modulated by the bandpass modulator 210. The digital bandpass transport system 402 includes a digitizing interface 405 that digitizes the bandpass signals, or bandpass waveform, into a digital stream. Digitizing is typically accomplished by using an analog-to-digital (A/D) converter that samples the bandpass waveform to produce digital bits included in the digital stream or, alternatively, the digitizing interface 405 may use other more complex signal processing systems. Subsequently, the digital stream is provided to a digital baseband transport segment 410. At the remote end of the digital baseband transport segment 410, a bandpass waveform regenerator 415 recovers the bandpass waveform and, subsequently, transmits the recovered analog bandpass signals across the conventional transmission segment 215 to the plurality of receivers 220. The bandpass waveform regenerator 415 may simply be a digital-to-analog (D/A) converter or other more complex signal processing systems.
Advantageously, like the communications system 300 in FIG. 3, the communications system 400 also allows for signal transport and transmission over greater distances than the system illustrated in FIG. 2. However, the communications system 400 does not require a bandpass modulator 210 at the end of every baseband transport segment 305 in the system where several distinct and separate transport segments exist. Disadvantageously, however, the signal quality for the regenerated bandpass signal is limited by the bit resolution of the digital sample stream provided by the digitizing interface 405. Furthermore, additional bandwidth is typically required to prevent signal aliasing according to the Nyquist sampling theory of the A/D converter, to accommodate the excess bandwidth or frequency deviation inherent to the chosen modulation scheme, or to provide guardband for practical filter implementation, to name but a few. The segment bandwidth, therefore, is very high relative to the bandwidth of the systems illustrated in FIGS. 2 and 3.
What is needed, therefore, is an improved communications system and method of transporting signals that focuses on the advantages existing in the previously mentioned system topologies while not degrading the performance in other areas. More specifically, a system is needed that has the advantages of greater transport segment distances, low signal bandwidth requirements, high signal quality, while also decreasing the system maintenance and expense.